No other single technique has had more impact on retinal investigation than fundus photography with the dye fluorescein. It has contributed to the diagnosis and treatment of chorioretinal diseases, elucidated poorly understood retinal diseases and served as an indispensable research modality.
Sodium fluorescein is a yellow-red dye with a molecular weight of 376.67. It is used in 10% or 25% sterile aqueous solution. When injected, usually into the antecubital vein, 80 % of it is bound to plasma proteins, mainly albumin, and the remainder stays free. When subjected to blue light (wave length 465-490 nm) it absorbs photons. The molecule then emits green-yellow light (wave length 520-530 nm). This capability is called fluorescence. The fluoresein is eventually metabolised in the kidney and liver and eliminated in 24-36 hours.
A specially designed fundus camera with appropriate light filters is used. The pupil has to be dilated in order to obtain optimal images and the patient's cooperation is extremely important. The dye should be injected rapidly to maximise the contrast of the early filling phase, with precaution to avoid extravasation of the dye, which is very painful and may lead to tissue necrosis. The skin turns yellow for a few hours and the patient's urine may have an orange hue for a few days.
The dye first enters the short posterior ciliary arteries and is visualised in the choroid and optic nerve head 10-15 sec after the injection. The dye enters the choroidal lobules sequentially and appears initially patchy on the photograph, but as the choroidal filling completes, it gives rise to the choroidal flush. The retinal circulation begins 1-3 sec after the onset of choroidal filling, i.e. 11-18 sec after the injection. The dye does not cross the normal blood retina barrier. It does not cross Bruch's membrane, which separates the choroids from the retina; neither does it extravasate normal capillaries. The arterial filling is complete in one second. The early arteriovenous phase follows, with the dye in the central retinal artery, the precapillary arteriols and the capillaries; the late arteriovenous phase is characterised by the passage of the dye through the veins in a laminar pattern, as well as by maximal fluorescence in the arteries.
Maximal fluorescence is achieved in the juxtafoveal or perifoveal capillary network in 20-25 sec, as in Figure 19.3. This is called the peak phase.
The normal capillary free zone is approximately 300-500 mu in diameter. The dark background to this zone in the macula is due to blockage of the choroidal fluorescence by the xanthophyll pigment and to the high density of the retinal pigment epithelial cells. The management of diabetic macular oedema requires an excellent peak phase series of images.
The first pass of fluorescein is complete at 30 sec and the dye recirculates giving rise to intermittent mild fluorescence. At 10 min the retinal and choroidal circulations are devoid of dye but the disc margins and the optic nerve head, Bruch's membrane, choroids and sclera remain stained.
Hypofluoresence is the term used whenever normal fluorescence is reduced either due to the masking effect of blood, exudate or any other retinal or choroidal pathology, such as chroidal naevus. It could also be due to vascular filling defect, as in retinal artery occlusion, or focal or extensive capillary drop out, as in proliferative diabetic retinopathy.
Hyperfluoresence is the term used whenever fluorescence is increased in an area either due to leakage, as in macular oedema, or due to a window defect that allows the choroidal fluorescence to show through an atrophic retina, as in macular degeneration. It can show as an area of pooling, as in pigment epithelial detachment (Rabb et al. 1978; Gass 1997).
In eyes with diabetes, fluorescein fundus angiography is particularly helpful in demonstrating leaking micro aneurysms, which helps in planning and carrying out laser treatment (Figure 19.4). It can demonstrate the status of the retinal vasculature, which may affect the decision for macula laser and pan retinal photocoagulation. From a research perspective, retinal circulation times, distribution of capillary filling and capillary permeability may each be quantified.
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